The present invention relates to a zoom lens, and more particularly relates to a zoom lens suitable for a compact video camera, wherein the angle of view of the zoom lens is not less than 60.degree. at the wide angle end and the magnification ratio is about 12.
In the case of a zoom lens for a compact video camera in which a solid image sensor is used, it has been desired to extend the wide angle range and also to increase the magnification ratio. In general, when the magnification ratio of this type of lens is increased, it is more difficult to extend the angle of view on the wide angle side than to extend the focal length on the telescopic side. The reason is that: as the angle of a lens is extended, the negative distortion on the wide angle side and astigmatism aberration of high order are increased, so that it becomes difficult to correct the aberration. When the lens angle is extended, the diameter of front lens is increased. Therefore, the weight and dimensions are increased, which does not meet the demand of the market. Accordingly, when a lens is designed, it is important to maintain the performance of the lens while expanding wide angle region and increasing magnification ratio and also it is important to make the lens compact.
In order to obtain a higher magnification zoom lens, a zoom lens including 5 groups having "positive", "negative", "positive", "negative" and "positive" refractive power is well known. This type of zoom lens is effective for correcting the distortion because the the lens is symmetrically structured. Therefore, the aforementioned lens structure is suitable to correct the distortion. The lens structure is disclosed in Japanese Patent Application Open to Public Inspection Nos. 2014/1982 and 39613/1985 in which all lens groups are moved for magnification change. The aforementioned lens structure is characterized in that: when the first lens group is moved to the object side, the magnification effect of the first and second lens groups can be improved; and when a space between the fourth and the fifth lens groups is varied, the image position is corrected in the process of zooming operation, so that the moving range of each lens group is small and a compact wide angle lens can be provided. However, in the aforementioned lens structure, the heaviest lens group is moved. Therefore, it is necessary to increase motor power in the power zooming system in which lens groups are driven by the motor. Further, a large-scale lens frame must be provided with a double structure composed of fixed and moving barrels. Accordingly, the aforementioned lens structure is disadvantageous in that: the dimensions of the mechanism are increased, so that the cost is raised. Furthermore, the aforementioned lens structure is disadvantageous in that: when the lens is protruded, the total lens length is increased, which causes a problem in lens design.
Further, Japanese Patent Publication No. 12125/1982 and Japanese Patent Application Open to Public Inspection No. 158813/1991 disclose a lens structure in which the first lens group is fixed. The aforementioned lens system includes: a first positive lens group that is fixed in the process of variable magnification; a second negative lens group that is moved on the optical axis in the process of magnification change; a third positive lens group; a fourth negative lens group that is fixed in the process of magnification change; and the fifth negative lens group that is also fixed in the process of magnification change. However, in the example shown in Japanese Patent Publication No. 12125/1982, the magnification ratio is 6, which is insufficient, and in the example shown in Japanese Patent Application Open to Public Inspection No. 158813/1991, the third lens group that is moved in the process of zooming, is provided with a diaphragm, so that the mechanism is complicated, which is disadvantageous.
When the first lens group is moved in a focusing operation, the front lens diameter is increased, so that the cost is raised, which is undesirable.